The invention relates to dosing in an injection apparatus for administering an injectable product in doses.
When administering products, in particular in medical applications, it is very important to dose the product precisely. In injection apparatus, the product is generally dosed with the aid of a dosing setting member which is coupled to a conveying means such that the dosage to be administered is selected with the aid of the dosing setting member and, when activated by the action of the conveying means, delivered. Through inappropriate handling, problems can arise from selecting the dosage. Problems which may be attributed to inappropriate handling occur to an increased extent in self-administering, as is for example common in the administering of insulin or growth hormone. Thus, a reaction movement by an element which is involved in dosing, caused by inappropriately performing dosing, can lead to incorrect dosing which is not noticed by the user.
An injection apparatus is known for example from EP 0 897 728 A1, in which the conveying means comprises a piston and a piston rod which acts on the piston and comprises a dosing thread. The piston rod is in threaded engagement with a dosing nut and in non-rotational engagement with a duplex dosage display sleeve for selecting the product dosage. The piston rod is mounted in a casing of the injection apparatus such that it can rotate about its longitudinal axis and move axially along the longitudinal axis. The dosing nut is mounted by the casing such that it can move axially along the rotational axis but cannot rotate about the rotational axis. A dosing rotational movement of the dosage display sleeve rotates the piston rod about its rotational axis and, due to the threaded engagement with the dosing nut in the casing, moves it axially in an advancing direction. The dosage display sleeve comprises two sleeve sections which can be rotated relative to each other, namely a sleeve section which displays the individual dosage units which can be selected and another sleeve section which displays the dosage units which can be selected in a division of ten. The two sleeve sections are connected non-rotationally to each other in discrete rotational positions with the aid of a split pin. The sleeve section with the division of ten can be rotated between two end positions, wherein in both end positions, a non-rotational connection to the sleeve section which displays the individual dosages exists due to the split pin. The non-rotational engagement between the piston rod and the dosage display sleeve is released when the piston rod assumes a front axial end position after performing a delivery stroke, and is re-established when the piston rod is returned to a rear axial end position. An unnoticed incorrect dosing can occur, for example, if the dosage display sleeve is situated in one of its end positions when the non-rotational engagement with the piston rod is established. The reason is that in order to establish the non-rotational connection, the engagement elements of the piston rod and the dosage display sleeve necessary for this have to “catch” each other, i.e., they are aligned with each other for the engagement.